There has been a remarkable growth in the large scale engineering applications of fiber reinforced plastics for over two decades. Such high performance materials, often referred to as composite materials, tend to possess preferred properties such as a relatively high strength and stiffness coupled with a relatively light weight. Specifically, the advantages of composite materials appear when the modulus per unit weight and strength per unit weight are considered. The tendency towards a higher specific modulus and specific strength in composites means that the weight of products incorporating them may be reduced.
Fiber reinforced plastics are typically comprised of a matrix polymer and a dispersed fiber phase, and are generally micro composites due to the small diameter of the fiber materials. Further, in fiber-filled composite materials, there are basically three regions: fiber, matrix and the interface between the fiber and matrix. The interfacial layers play an important role in the Theological and mechanical properties of composites since stresses acting on the matrix are transmitted to the fiber across the interface. Only with an effective transmittal of those stresses can the greater strength properties of the fiber be exploited. Therefore, good adhesion between the matrix and fiber is required in order to achieve the desired mechanical properties.
However, there is a tendency for fiber-reinforced plastics to exhibit poor adhesion between the matrix and fiber or filler particle surfaces as a result of their dissimilar natures. For example, the filler or reinforcing agent is usually hydrophilic and the matrix is generally hydrophobic. Moreover, when the fiber is hydrophilic, it can adsorb water, which tends to decrease the impact resistance of the composite material significantly. For example, a matrix such as Nylon 6 (trademark), due to its moisture sensitivity, can allow water molecules to diffuse and reach the fiber surface, thereby resulting in delamination of the interface. Thus, in such materials, adhesion between the fiber and the matrix is generally poor.
Many attempts have been made to improve the interfacial adhesion between the fiber surface and the matrix in order to produce a high performance composite material with superior properties. Such attempts include etching of the fibers, plasma treatment of the fibers, the use of a coupling or compatibilizing agent, and the use of block or graft copolymers.
A number of copolymers of .epsilon.-caprolactam and non carbonate co-monomers have been synthesized by several researchers. Copolymers are defined as polymer chains containing two or more repeat units chemically linked to each other in some way. Depending on the sequence of these different repeat units, copolymers can be further classified into various types, viz. random copolymers, alternating copolymers, graft copolymers and block copolymers.
Block copolymers are comprised of chemically dissimilar, chemically connected segments. Their sequential arrangement can vary from "A-B" type structures, containing two segments only (di-block copolymers), to "A-B-A" block copolymers with three segments (tri-block copolymers), to multi-block "(-A-B-).sub.n " systems possessing many segments (multi-block copolymers). Block copolymers usually exhibit improved interphase adhesion resulting from the microphase morphology of the copolymer, which can produce a relatively good balance of mechanical properties.
The strategy generally adopted for synthesizing these copolymers is an outcome of the nature of the mechanism of polymerization of .epsilon.-caprolactam to Nylon 6.TM., as described further below. In the synthesis of Nylon 6.TM., an N-acyllactam is necessary for the initiation of the chain. Thus, if a preformed polymer (which is to be copolymerized with Nylon 6.TM.) is capped at either one or both ends with a lactam unit in such a way that an active N-acyllactam is produced, this polymer, which is actually an N-acyllactam, could serve as an activator in the polymerization of Nylon 6.TM.. Consequently, .epsilon.-caprolactam could be polymerized from the end of the polymeric activator by the usual ring-opening technique, thereby producing a block copolymer of the two homopolymers.
Using this technique, Keul H. et al., European Polumer Journal, 28(6), 611(1992) made an attempt to synthesize an A-B block polymer with an aliphatic polycarbonate block (A) and a Nylon 6.TM. block (B). Their procedure contemplated capping of the "living" poly(2,2-dimethyltrimethylene carbonate) chains with .epsilon.-caprolactam moieties to give an N-acyllactam. However, they discovered that alkali metal-based catalysts fail to polymerize .epsilon.-caprolactam in the presence of the aliphatic carbonate chain.
Wurm B. et al. in Macromolecules, 25 2977 (1992) and in Makromol. Chem., Rapid Commun., 13, 9 (1992) attempted to employ a different strategy to synthesize poly(2,2-dimethyltrimethylene carbonate)-Nylon 6.TM. block copolymers. They used "living" poly(2,2-dimethyltrimethylene carbonate) chains as initiators, unlike the above (wherein polymers were capped to give activators), to polymerize .epsilon.-caprolactam in order to yield the desired block copolymers. This route is well-known for making block copolymers of poly(2,2-dimethyltrimethylene carbonate) with certain other polymers. However, their study showed that alternating, rather than block, copolymers of these two homopolymers were obtained by this process.
Further, sodium hydride is a commonly used initiator in the polymerization of .epsilon.-caprolactam to Nylon 6.TM., both commercially and for research purposes. However, sodium hydride is known to hinder the polymerization of .epsilon.-caprolactam in the presence of aliphatic carbonates [Krimm H. et. al. Chemical Abstracts, 97, 93020x (1982) and Krimm H. et. al. Chemical Astracts, 97, 56407h (1982)].
Keul H. et. al. similarly showed that in the presence of an aliphatic polycarbonate, the active species (the metal caprolactamate) in the polymerization of .epsilon.-caprolactam, with alkali metals as counterions, reacts with a carbonate group of the carbonate monomer or polymer rather than with an activated .epsilon.-caprolactam moiety. This leads to breaking down of the polycarbonate chains and simultaneous consumption of the initiator. Eventually all of the initiator is used up so that there is no possibility of any polymerization taking place.
Blending of two or more polymers also provides another route for the development of new materials for engineering applications. Polymer blends are defined as mixtures of two or more polymers or copolymers in which the individual polymer chains do not react with each other chemically. Polymer blends can be homogeneous (miscible) or heterogeneous (immiscible), although the vast majority of blended composites consist of pairs or groups of immiscible polymers. This means that the product is not a homogeneous, single-phase material but is composed of a matrix material and one or more dispersed phases.
Ideally, two or more polymers may be blended to form a wide variety of random or structured morphologies to obtain products that potentially offer desirable combinations of characteristics. However, it is often difficult or impossible in practice to achieve these potential combinations through simple mechanical blending. As mentioned earlier, the two polymers are frequently thermodynamically immiscible, which precludes generating a truly homogeneous product.
These problems may be alleviated by the presence of certain polymeric species, such as a block or graft copolymer, suitably chosen. It is generally believed that this is a result of their ability to alter the interfacial situation. Such species, as a consequence, are often referred to as "compatibilizers", which is analogous to the term "solubilizers" used in the colloid field to describe the effect surfactants have on the ability to "mix" oil and water. The general view is that a properly chosen block or graft copolymer can preferentially locate at the interface between the two phases. As well, newer technology provides for reactive blending. This technique involves in situ reaction between the homopolymers by means of adding a reactive ingredient: such as ionomers, adducts of maleic or fumaric acids (or their anhydrides), or succinic copolymers.
Cortazar M. et. al., British Polymer Journal, 21, 395 (1989) have theorized that interchange reactions may occur in the Nylon 6.TM./polycarbonate system in a high temperature melt state. In their investigation, a blend of composition 50/50 was originally prepared by a solution blending process at room temperature using phenol/methanol and later maintained in the melt state at 250.degree. C. in a calorimetric pan under a nitrogen atmosphere for different periods of time. These were then analyzed with the help of calorimetric analyses by cooling and reheating at a controlled rate. The heats of melting and crystallization and the respective temperatures of these transitions appeared to drop with increasing reaction time in the pan. This suggested that interchange reactions may have taken place in the blend during heat treatment.
Gattiglia E. et. al., Journal of Applied Polymer Science, 38, 1807 (1989) prepared blends of Nylon 6.TM. and polycarbonate by shearing the polymers in a single screw extruder at 250.degree. C. They studied the thermal properties and morphology of the system over the entire composition range. Also, blend samples were treated with solvent and the extracts were analyzed using gel permeation chromatography (GPC) after they were sheared through the extruder. The GPC analysis showed degradation of polycarbonate which may have been due to reactions taking place in the systems. The Nylon 6.TM. melting point as well as polycarbonate glass transition temperature decreased with increasing polycarbonate concentration in the mixture. The scanning electron micrographs showed that the blends were immiscible at all compositions except for the one which had 95% Nylon 6.TM.. As a result, it was theorized that reactions taking place in these blends may be responsible for degradative effects observed for polycarbonate.
In the second part of their work, Gattiglia E. et. al., Journal of Applied Polymer Science, 41, 1411 (1990) investigated the morphology-mechanical property relationships in the above blends. The impact strength of polycarbonate dropped by a factor of 10 on addition of 5%, Nylon 6.TM. whereas that of Nylon 6.TM. increased by a factor of 2 when 5% or 10% polycarbonate was present. This suggested that compatibilization due to chemical reactions may take place in the system at higher Nylon 6.TM. concentrations. Other mechanical properties of the blends were generally found to be poorer than the parent polymers themselves.
In all the above experiments by Gattiglia E. et. al., however, the effect of shearing time on the blend morphology and mechanical properties was not specifically studied. In Gattiglia E. et. al., Journal of Applied Polymer Science, 46, 1887 (1992), they used a batch mixer in place of an extruder and varied the time of blending for these blends. Their results showed that following longer periods of mixing, the blends appeared to demonstrate improved compatibility. Nonetheless, the mechanical properties did not show any improvement over those of the pure homopolymers except for the tensile modulus of a blend with 90% Nylon 6.TM.. This was attributed to the compensation that occurred due to fragmentation of polycarbonate chains during the course of the mixing, i.e. greater degradation of polycarbonate chains resulted from longer blending times.
All the above work shows that Nylon 6.TM. and polycarbonate are immiscible polymers which may show some compatibility due to reactions possibly taking place in site during high-temperature shearing of the two polymers together. However, the improvement of mechanical properties, the ultimate goal of mixing, has to date not been achieved to the desired extent in such blends.
As a result, there is a need in the industry for a process for the production of a polymerized material having improved mechanical properties as compared to known polymerized materials. Further, there is a need for a process for the production of a polymerized material from an organic amide monomer, preferably .epsilon.-caprolactam, and an organic carbonate, preferably selected from the group consisting of polycarbonates, cyclic oligomers and mixtures thereof. There is also a need for a polymerized material having improved mechanical properties as compared to known polymerized materials. As well, there is a need for a polymerized material produced by the polymerization of an organic amide monomer, preferably .epsilon.-caprolactam, and an organic carbonate, preferably selected from the group consisting of polycarbonates, cyclic oligomers and mixtures thereof.